The present invention relates to improvements of a negative electrode of non-aqueous electrolyte secondary batteries. More specifically, this invention relates to a non-aqueous electrolyte secondary battery having a high electric capacity, having a highly reliable negative electrode free from dendrites, and offering a high energy density without a fear of occurrence of internal short circuit caused by dendrites.
Non-aqueous electrolyte secondary batteries having a negative electrode of lithium or lithium compounds have been attracting a great deal of attention because they afford a high discharge voltage and a high energy density, and therefore extensive investigations have been made about such batteries.
Conventionally known positive electrode active materials for use in the non-aqueous electrolyte secondary battery include oxides or chalcogenides of transition metals such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2, MoS2 and the like. These compounds have a layered or tunneled crystal structure permitting intercalation and deintercalation of lithium ions. On the other hand, as a negative electrode active material, the use of metallic lithium has been investigated extensively. However, metallic lithium, when used in the negative electrode, has an inevitable issue of deposition of lithium dendrites on the surface of metallic lithium during the charge, which impairs charge and discharge efficiency or induces internal short circuit due to the contact of such lithium dendrites with the positive electrode. At the present time, there have been put into practice lithium ion batteries using, in the negative electrode, a graphite type carbon material which is capable of reversibly absorbing therein and desorbing therefrom lithium and which is excellent in terms of cycle characteristics and safety although it has smaller capacity than metallic lithium.
However, when a graphite material is used in the negative electrode, its theoretical capacity is 372 mAh/g, which is about one tenth of metallic lithium, and its theoretical density is as low as 2.2 g/cc. When the material is made into a negative electrode sheet in practice, the density decreases further. For such a reason, the use of a material having a higher capacity per volume is desired with a view to realizing a battery with a higher capacity.
Under the circumstances, proposals have been made about the use of oxides in the negative electrode in order to realize a further higher capacity. For example, there is proposed the use of amorphous oxides such as SnSiO3 and SnSi1-xPxO3 in the negative electrode in order to improve cycle characteristics (Japanese Laid-Open Patent Publication No. Hei 7-288123). However, when such oxides are used in the negative electrode, the difference between the initial charge capacity and the discharge capacity, i.e., irreversible capacity, is extremely large, which makes it difficult to put this into practical use.
On the other hand, various proposals have also been made about the use of alloy materials in the negative electrode. For example, there is proposed the use of inter-metallic compounds formed by alloying a metallic element which is electrochemically inactive with lithium, such as iron and nickel, together with an element which can electrochemically alloy with lithium, such as aluminum and tin (Japanese Laid-Open Patent Publication No. Hei 10-223221). With this measure, a negative electrode material having an extremely high capacity is supposed to be achieved. Nevertheless, even with this measure, the short cycle life and the expansion of the negative electrode mixture caused by the intercalation of lithium cannot be avoided, which makes it difficult to put this into practical use.
In view of the problems as described above, the present invention has an object to provide a negative electrode which permits a non-aqueous electrolyte secondary battery having a high capacity and excellent charge and discharge cycle characteristics.
The present invention provides a negative electrode comprising an alloy material which absorbs lithium during charge and desorbs lithium during discharge, developing no dendrites, offering a high electric capacity, suffering little expansion during the charge, and having a long cycle life.
The present invention provides a non-aqueous electrolyte secondary battery comprising a rechargeable positive electrode, a non-aqueous electrolyte containing a lithium salt and a rechargeable negative electrode, wherein the negative electrode includes an alloy having a hexagonal closest packing structure and a Ni2In type structure composed of at least two elements.
Herein, the aforementioned alloy preferably comprises an intermetallic compound containing at least one element selected from the group consisting of Sn, Si and In.
The aforementioned alloy preferably comprises at least one intermetallic compound selected from the group consisting of Ti2Sn, NiCoSn, Mn2Sn, Ni3Sn2, BeSiZr, Co3Sn2, Cu2In, Ni2In, Ni2Si, Pd3Sn2 and Rh3Sn2.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.